diff --git a/docs/platform-feature-matrix.json b/docs/platform-feature-matrix.json
new file mode 100644
index 000000000..42fc432fa
--- /dev/null
+++ b/docs/platform-feature-matrix.json
@@ -0,0 +1,329 @@
+{
+  "schema_version": 1,
+  "updated": "2026-07-06",
+  "source_issues": [
+    "https://github.com/headroomlabs-ai/headroom/issues/1843"
+  ],
+  "status_values": [
+    "covered",
+    "partial",
+    "gap",
+    "blocked"
+  ],
+  "platforms": [
+    "linux",
+    "macos",
+    "windows"
+  ],
+  "features": [
+    {
+      "id": "install_apply_python",
+      "name": "Persistent Python install applies and starts",
+      "risk": "install",
+      "platforms": {
+        "linux": {
+          "status": "covered",
+          "tests": [
+            "tests/test_cli/test_install_cli.py",
+            "tests/test_install/test_supervisors.py",
+            ".github/workflows/install-native-e2e.yml"
+          ]
+        },
+        "macos": {
+          "status": "covered",
+          "tests": [
+            "tests/test_install/test_supervisors.py",
+            ".github/workflows/install-native-e2e.yml"
+          ]
+        },
+        "windows": {
+          "status": "partial",
+          "tests": [
+            "tests/test_install/test_supervisors.py",
+            "tests/test_install/test_runtime.py",
+            ".github/workflows/ci.yml#windows-native-wrapper"
+          ],
+          "gap": "Native install smoke workflow is source-level until Windows wheel CRT conflicts are resolved."
+        }
+      }
+    },
+    {
+      "id": "install_windows_service",
+      "name": "Windows service install uses sc.exe safely",
+      "risk": "install",
+      "platforms": {
+        "linux": {
+          "status": "covered",
+          "tests": [
+            "tests/test_install/test_supervisors.py"
+          ]
+        },
+        "macos": {
+          "status": "covered",
+          "tests": [
+            "tests/test_install/test_supervisors.py"
+          ]
+        },
+        "windows": {
+          "status": "covered",
+          "tests": [
+            "tests/test_install/test_supervisors.py"
+          ]
+        }
+      }
+    },
+    {
+      "id": "single_instance_start",
+      "name": "Persistent runtime start is single-instance by default",
+      "risk": "runtime",
+      "platforms": {
+        "linux": {
+          "status": "covered",
+          "tests": [
+            "tests/test_cli/test_install_cli.py",
+            "tests/test_install/test_runtime.py"
+          ]
+        },
+        "macos": {
+          "status": "covered",
+          "tests": [
+            "tests/test_cli/test_install_cli.py",
+            "tests/test_install/test_runtime.py"
+          ]
+        },
+        "windows": {
+          "status": "covered",
+          "tests": [
+            "tests/test_cli/test_install_cli.py",
+            "tests/test_install/test_runtime.py"
+          ]
+        }
+      }
+    },
+    {
+      "id": "compression_fail_open",
+      "name": "Slow or saturated compression fails open",
+      "risk": "performance",
+      "platforms": {
+        "linux": {
+          "status": "covered",
+          "tests": [
+            "tests/test_platform_stabilization_functional.py",
+            "tests/test_kompress_request_nonblocking.py",
+            "tests/test_proxy_compression_executor.py",
+            "tests/test_openai_codex_ws_lifecycle.py"
+          ]
+        },
+        "macos": {
+          "status": "partial",
+          "tests": [
+            "tests/test_platform_stabilization_functional.py",
+            "tests/test_kompress_request_nonblocking.py",
+            "tests/test_proxy_compression_executor.py"
+          ],
+          "gap": "Native workflow coverage should add focused performance-gate tests without full model downloads."
+        },
+        "windows": {
+          "status": "partial",
+          "tests": [
+            "tests/test_platform_stabilization_functional.py",
+            "tests/test_kompress_request_nonblocking.py",
+            "tests/test_proxy_compression_executor.py"
+          ],
+          "gap": "Source-level tests validate fail-open semantics; full native proxy e2e waits on Windows wheel availability."
+        }
+      }
+    },
+    {
+      "id": "proxy_functional_smoke",
+      "name": "Proxy health and /v1/compress work through the app surface",
+      "risk": "proxy",
+      "platforms": {
+        "linux": {
+          "status": "covered",
+          "tests": [
+            "tests/test_platform_stabilization_functional.py",
+            "tests/test_ccr_row_drop_store_bridge.py"
+          ]
+        },
+        "macos": {
+          "status": "partial",
+          "tests": [
+            "tests/test_platform_stabilization_functional.py",
+            "tests/test_ccr_row_drop_store_bridge.py"
+          ],
+          "gap": "FastAPI route coverage exists; native persistent proxy process smoke should be added."
+        },
+        "windows": {
+          "status": "partial",
+          "tests": [
+            "tests/test_platform_stabilization_functional.py",
+            "tests/test_ccr_row_drop_store_bridge.py"
+          ],
+          "gap": "FastAPI route coverage exists; native persistent proxy process smoke waits on Windows wheel availability."
+        }
+      }
+    },
+    {
+      "id": "ccr_persistence",
+      "name": "CCR survives restart when persistent storage is enabled",
+      "risk": "cache",
+      "platforms": {
+        "linux": {
+          "status": "covered",
+          "tests": [
+            "crates/headroom-core/tests/ccr_backends.rs",
+            "tests/test_storage_backends.py"
+          ]
+        },
+        "macos": {
+          "status": "partial",
+          "tests": [
+            "crates/headroom-core/tests/ccr_backends.rs",
+            "tests/test_storage_backends.py"
+          ],
+          "gap": "Rust backend tests run in the Rust workflow; native macOS restart e2e is not yet present."
+        },
+        "windows": {
+          "status": "partial",
+          "tests": [
+            "crates/headroom-core/tests/ccr_backends.rs",
+            "tests/test_storage_backends.py"
+          ],
+          "gap": "Windows restart e2e is blocked by the native wheel CRT conflict."
+        }
+      }
+    },
+    {
+      "id": "init_cli",
+      "name": "headroom init configures supported agents",
+      "risk": "install",
+      "platforms": {
+        "linux": {
+          "status": "covered",
+          "tests": [
+            ".github/workflows/init-native-e2e.yml"
+          ]
+        },
+        "macos": {
+          "status": "covered",
+          "tests": [
+            ".github/workflows/init-native-e2e.yml"
+          ]
+        },
+        "windows": {
+          "status": "blocked",
+          "tests": [
+            ".github/workflows/init-native-e2e.yml"
+          ],
+          "gap": "Matrix entry is intentionally excluded until Windows wheel CRT conflicts are resolved."
+        }
+      }
+    },
+    {
+      "id": "wrap_prepare_only",
+      "name": "headroom wrap prepare-only mutates config safely",
+      "risk": "install",
+      "platforms": {
+        "linux": {
+          "status": "covered",
+          "tests": [
+            ".github/workflows/wrap-native-e2e.yml"
+          ]
+        },
+        "macos": {
+          "status": "covered",
+          "tests": [
+            ".github/workflows/wrap-native-e2e.yml"
+          ]
+        },
+        "windows": {
+          "status": "blocked",
+          "tests": [
+            ".github/workflows/wrap-native-e2e.yml"
+          ],
+          "gap": "Matrix entry is intentionally excluded until Windows wheel CRT conflicts are resolved."
+        }
+      }
+    },
+    {
+      "id": "toin_skip_recommendations",
+      "name": "TOIN skip-compression recommendations are exposed",
+      "risk": "cache",
+      "platforms": {
+        "linux": {
+          "status": "covered",
+          "tests": [
+            "tests/test_toin.py",
+            "tests/test_proxy_ccr.py",
+            "tests/test_compression_policy_toin_gate.py"
+          ]
+        },
+        "macos": {
+          "status": "partial",
+          "tests": [
+            "tests/test_toin.py",
+            "tests/test_compression_policy_toin_gate.py"
+          ],
+          "gap": "Native end-to-end verification of served recommendations is still needed."
+        },
+        "windows": {
+          "status": "partial",
+          "tests": [
+            "tests/test_toin.py",
+            "tests/test_compression_policy_toin_gate.py"
+          ],
+          "gap": "Native end-to-end verification is blocked by Windows wheel availability."
+        }
+      }
+    }
+  ],
+  "sanity_tests": [
+    {
+      "id": "cli_help",
+      "description": "Every public CLI command renders help without importing optional heavy runtimes.",
+      "tests": [
+        "tests/test_cli"
+      ]
+    },
+    {
+      "id": "install_paths",
+      "description": "Install paths resolve under the active user profile and never require administrator paths for user scope.",
+      "tests": [
+        "tests/test_install/test_paths.py"
+      ]
+    },
+    {
+      "id": "runtime_selection",
+      "description": "Python, Docker, service, and task runtime commands are generated deterministically for each platform.",
+      "tests": [
+        "tests/test_install/test_runtime.py",
+        "tests/test_install/test_supervisors.py"
+      ]
+    },
+    {
+      "id": "health_startup",
+      "description": "Persistent starts wait for /readyz and surface startup failure instead of silently succeeding.",
+      "tests": [
+        "tests/test_cli/test_install_cli.py",
+        "tests/test_install/test_health.py"
+      ]
+    },
+    {
+      "id": "compression_backpressure",
+      "description": "Compression queue saturation and model cold-start fail open without hanging request paths.",
+      "tests": [
+        "tests/test_platform_stabilization_functional.py",
+        "tests/test_kompress_request_nonblocking.py",
+        "tests/test_proxy_compression_executor.py"
+      ]
+    },
+    {
+      "id": "proxy_route_smoke",
+      "description": "Health and /v1/compress routes return functional metrics and fail open on timeout.",
+      "tests": [
+        "tests/test_platform_stabilization_functional.py"
+      ]
+    }
+  ]
+}
diff --git a/docs/platform-stabilization.md b/docs/platform-stabilization.md
new file mode 100644
index 000000000..bb3c63f42
--- /dev/null
+++ b/docs/platform-stabilization.md
@@ -0,0 +1,28 @@
+# Platform Stabilization Matrix
+
+This matrix is the hardening source of truth for install, startup, runtime, cache, and compression behavior across Linux, macOS, and Windows. The machine-readable matrix lives in `docs/platform-feature-matrix.json`; CI tests validate its shape so every feature has explicit platform status and test evidence.
+
+## Status Values
+
+- `covered`: unit, integration, or native e2e coverage exists for the platform.
+- `partial`: coverage exists, but a named gap remains.
+- `gap`: no meaningful coverage exists yet.
+- `blocked`: coverage is intentionally excluded by a known external blocker.
+
+## Current Priorities
+
+1. Keep install/setup/run idempotent. Persistent starts must not create duplicate proxy instances unless a future explicit opt-in exists.
+2. Keep compression fail-open. Cold model downloads, saturated executors, or timeout paths must pass through unchanged traffic instead of hanging agents.
+3. Keep cache behavior visible. CCR persistence and TOIN skip recommendations must have restart and served-recommendation coverage.
+4. Keep Windows honest. Windows-specific tests should run locally and in CI whenever they do not require the currently blocked native wheel build.
+
+## Issue 1843 Coverage Map
+
+- Windows service quoting: `install_windows_service`
+- Duplicate startup processes: `single_instance_start`
+- Slow compression and hangs: `compression_fail_open`
+- CCR restart persistence: `ccr_persistence`
+- TOIN recommendation wiring: `toin_skip_recommendations`
+- Install/setup/run sanity: `install_apply_python`, `init_cli`, `wrap_prepare_only`
+
+When adding or closing a hardening item, update `docs/platform-feature-matrix.json` in the same PR as the implementation or test change.
diff --git a/headroom/cli/install.py b/headroom/cli/install.py
index 66221fd36..249550d27 100644
--- a/headroom/cli/install.py
+++ b/headroom/cli/install.py
@@ -59,11 +59,26 @@ def _require_manifest(profile: str) -> DeploymentManifest:
     return manifest
 
 
-def _start_deployment(manifest: DeploymentManifest) -> None:
+def _start_deployment(manifest: DeploymentManifest, *, assume_start_lock: bool = False) -> None:
+    if not assume_start_lock:
+        with acquire_runtime_start_lock(manifest.profile) as acquired:
+            if not acquired:
+                click.echo(f"Deployment '{manifest.profile}' start is already in progress.")
+                return
+            _start_deployment(manifest, assume_start_lock=True)
+            return
+
+    if probe_ready(manifest.health_url):
+        return
     if manifest.preset == InstallPreset.PERSISTENT_DOCKER.value and shutil.which("docker") is None:
         raise click.ClickException(
             "Docker is required for this deployment but 'docker' was not found on PATH."
         )
+    if runtime_status(manifest) == "running":
+        if wait_ready(manifest, timeout_seconds=_STARTUP_READY_TIMEOUT_SECONDS):
+            return
+        stop_runtime(manifest)
+
     try:
         if manifest.preset == InstallPreset.PERSISTENT_DOCKER.value:
             start_persistent_docker(manifest)
@@ -77,7 +92,7 @@ def _start_deployment(manifest: DeploymentManifest) -> None:
     except subprocess.CalledProcessError as e:
         raise click.ClickException(
             f"Cannot start deployment '{manifest.profile}': command failed "
-            f"({' '.join(map(str, e.cmd)) if isinstance(e.cmd, (list, tuple)) else e.cmd})"
+            f"({' '.join(map(str, e.cmd)) if isinstance(e.cmd, list | tuple) else e.cmd})"
         ) from None
 
     if not wait_ready(manifest, timeout_seconds=45):
@@ -276,7 +291,7 @@ def install_apply(
             _restore_deployment(existing)
         # Surface non-Click errors (OSError, CalledProcessError, …) as a clean
         # message rather than a raw traceback; Click errors pass through as-is.
-        if isinstance(exc, (click.ClickException, click.Abort)):
+        if isinstance(exc, click.ClickException | click.Abort):
             raise
         raise click.ClickException(f"Failed to install deployment '{profile}': {exc}") from exc
 
@@ -409,5 +424,5 @@ def install_agent_ensure(profile: str) -> None:
                 click.echo(f"Deployment '{profile}' is healthy.")
                 return
             stop_runtime(manifest)
-        _start_deployment(manifest)
+        _start_deployment(manifest, assume_start_lock=True)
     click.echo(f"Deployment '{profile}' is healthy.")
diff --git a/headroom/proxy/handlers/openai.py b/headroom/proxy/handlers/openai.py
index dbff4e627..9ec52bf79 100644
--- a/headroom/proxy/handlers/openai.py
+++ b/headroom/proxy/handlers/openai.py
@@ -7218,19 +7218,21 @@ class OpenAIHandlerMixin:
             )
         except TimeoutError:
             logger.warning(
-                "Compression timed out after %.0fs (payload too large)",
+                "Compression timed out after %.0fs; failing open with original messages",
                 COMPRESSION_TIMEOUT_SECONDS,
             )
             return JSONResponse(
-                status_code=503,
                 content={
-                    "error": {
-                        "type": "compression_timeout",
-                        "message": (
-                            "Compression exceeded "
-                            f"{COMPRESSION_TIMEOUT_SECONDS:.0f}s; payload too large."
-                        ),
-                    }
+                    "messages": messages,
+                    "tokens_before": 0,
+                    "tokens_after": 0,
+                    "tokens_saved": 0,
+                    "compression_ratio": 1.0,
+                    "transforms_applied": [],
+                    "transforms_summary": {},
+                    "ccr_hashes": [],
+                    "compression_skipped": True,
+                    "skip_reason": "compression_timeout",
                 },
             )
         except Exception as e:
diff --git a/tests/test_cli/test_install_cli.py b/tests/test_cli/test_install_cli.py
index 2ba8b5577..6cccaeb92 100644
--- a/tests/test_cli/test_install_cli.py
+++ b/tests/test_cli/test_install_cli.py
@@ -43,6 +43,8 @@ def test_install_apply_starts_service_supervisor(monkeypatch) -> None:
     monkeypatch.setattr(
         "headroom.cli.install.wait_ready", lambda deployment, timeout_seconds=45: True
     )
+    monkeypatch.setattr("headroom.cli.install.probe_ready", lambda url: False)
+    monkeypatch.setattr("headroom.cli.install.runtime_status", lambda manifest: "stopped")
 
     result = runner.invoke(main, ["install", "apply"])
 
@@ -153,6 +155,8 @@ def test_install_restart_uses_internal_helpers(monkeypatch) -> None:
     monkeypatch.setattr(
         "headroom.cli.install.wait_ready", lambda manifest, timeout_seconds=45: True
     )
+    monkeypatch.setattr("headroom.cli.install.probe_ready", lambda url: False)
+    monkeypatch.setattr("headroom.cli.install.runtime_status", lambda manifest: "stopped")
 
     result = runner.invoke(main, ["install", "restart"])
 
@@ -161,6 +165,114 @@ def test_install_restart_uses_internal_helpers(monkeypatch) -> None:
     assert calls == ["stop_supervisor", "stop_runtime", "start_supervisor"]
 
 
+def test_install_start_noops_when_already_healthy(monkeypatch) -> None:
+    runner = CliRunner()
+    calls: list[str] = []
+
+    class Manifest:
+        profile = "default"
+        preset = "persistent-service"
+        runtime_kind = "python"
+        supervisor_kind = "service"
+        scope = "user"
+        health_url = "http://127.0.0.1:8787/readyz"
+
+    monkeypatch.setattr("headroom.cli.install.load_manifest", lambda profile: Manifest())
+    monkeypatch.setattr("headroom.cli.install.probe_ready", lambda url: True)
+    monkeypatch.setattr(
+        "headroom.cli.install.start_supervisor", lambda manifest: calls.append("start_supervisor")
+    )
+
+    result = runner.invoke(main, ["install", "start"])
+
+    assert result.exit_code == 0, result.output
+    assert "Started deployment 'default'." in result.output
+    assert calls == []
+
+
+def test_install_start_noops_for_healthy_docker_without_docker_on_path(monkeypatch) -> None:
+    runner = CliRunner()
+
+    class Manifest:
+        profile = "default"
+        preset = "persistent-docker"
+        runtime_kind = "docker"
+        supervisor_kind = "none"
+        scope = "user"
+        health_url = "http://127.0.0.1:8787/readyz"
+
+    monkeypatch.setattr("headroom.cli.install.load_manifest", lambda profile: Manifest())
+    monkeypatch.setattr("headroom.cli.install.probe_ready", lambda url: True)
+    monkeypatch.setattr("headroom.cli.install.shutil.which", lambda name, *args, **kwargs: None)
+
+    result = runner.invoke(main, ["install", "start"])
+
+    assert result.exit_code == 0, result.output
+    assert "Started deployment 'default'." in result.output
+
+
+def test_install_start_does_not_spawn_when_start_lock_is_contended(monkeypatch) -> None:
+    runner = CliRunner()
+    calls: list[str] = []
+
+    class Manifest:
+        profile = "default"
+        preset = "persistent-service"
+        runtime_kind = "python"
+        supervisor_kind = "service"
+        scope = "user"
+        health_url = "http://127.0.0.1:8787/readyz"
+
+    monkeypatch.setattr("headroom.cli.install.load_manifest", lambda profile: Manifest())
+
+    import contextlib
+
+    @contextlib.contextmanager
+    def fake_lock(profile):
+        yield False
+
+    monkeypatch.setattr("headroom.cli.install.acquire_runtime_start_lock", fake_lock)
+    monkeypatch.setattr(
+        "headroom.cli.install.start_supervisor", lambda manifest: calls.append("start_supervisor")
+    )
+
+    result = runner.invoke(main, ["install", "start"])
+
+    assert result.exit_code == 0, result.output
+    assert "start is already in progress" in result.output
+    assert calls == []
+
+
+def test_install_start_restarts_wedged_runtime_under_single_lock(monkeypatch) -> None:
+    runner = CliRunner()
+    calls: list[str] = []
+
+    class Manifest:
+        profile = "default"
+        preset = "persistent-service"
+        runtime_kind = "python"
+        supervisor_kind = "service"
+        scope = "user"
+        health_url = "http://127.0.0.1:8787/readyz"
+
+    monkeypatch.setattr("headroom.cli.install.load_manifest", lambda profile: Manifest())
+    monkeypatch.setattr("headroom.cli.install.probe_ready", lambda url: False)
+    monkeypatch.setattr("headroom.cli.install.runtime_status", lambda manifest: "running")
+    wait_results = iter([False, True])
+    monkeypatch.setattr(
+        "headroom.cli.install.wait_ready", lambda manifest, timeout_seconds: next(wait_results)
+    )
+    monkeypatch.setattr("headroom.cli.install.stop_runtime", lambda manifest: calls.append("stop"))
+    monkeypatch.setattr(
+        "headroom.cli.install.start_supervisor", lambda manifest: calls.append("start_supervisor")
+    )
+
+    result = runner.invoke(main, ["install", "start"])
+
+    assert result.exit_code == 0, result.output
+    assert calls == ["stop", "start_supervisor"]
+
+
 def test_install_apply_rejects_invalid_profile() -> None:
     runner = CliRunner()
 
@@ -350,6 +462,7 @@ def test_install_apply_uses_docker_runtime_for_persistent_docker(monkeypatch) ->
         supervisor_kind = "none"
         scope = "user"
         health_url = "http://127.0.0.1:8787/readyz"
+        container_name = "headroom-default"
         targets: list[str] = []
         mutations = []
         artifacts = []
@@ -366,6 +479,8 @@ def test_install_apply_uses_docker_runtime_for_persistent_docker(monkeypatch) ->
     monkeypatch.setattr(
         "headroom.cli.install.wait_ready", lambda deployment, timeout_seconds=45: True
     )
+    monkeypatch.setattr("headroom.cli.install.probe_ready", lambda url: False)
+    monkeypatch.setattr("headroom.cli.install.runtime_status", lambda deployment: "stopped")
     # _start_deployment guards the persistent-docker preset with
     # `shutil.which("docker")`. Fake docker as present so the test exercises the
     # runtime-selection path itself rather than the host's docker install —
@@ -517,7 +632,8 @@ def test_install_agent_ensure_stops_wedged_runtime_before_restart(monkeypatch) -
 
     monkeypatch.setattr("headroom.cli.install.acquire_runtime_start_lock", fake_lock)
     monkeypatch.setattr(
-        "headroom.cli.install._start_deployment", lambda manifest: calls.append("start_deployment")
+        "headroom.cli.install._start_deployment",
+        lambda manifest, **kwargs: calls.append("start_deployment"),
     )
 
     result = runner.invoke(main, ["install", "agent", "ensure"])
@@ -626,7 +742,7 @@ def test_install_agent_ensure_propagates_start_deployment_failure(monkeypatch) -
 
     monkeypatch.setattr("headroom.cli.install.acquire_runtime_start_lock", fake_lock)
 
-    def boom(manifest):
+    def boom(manifest, **kwargs):
         raise click.ClickException("simulated start failure")
 
     monkeypatch.setattr("headroom.cli.install._start_deployment", boom)
diff --git a/tests/test_install/test_runtime.py b/tests/test_install/test_runtime.py
index c9f144fc4..f15cda93b 100644
--- a/tests/test_install/test_runtime.py
+++ b/tests/test_install/test_runtime.py
@@ -272,7 +272,12 @@ def test_runtime_start_lock_blocks_another_process(monkeypatch, tmp_path: Path)
         "with acquire_runtime_start_lock('default') as acquired:\n"
         "    print(acquired)\n"
     )
-    env = {**os.environ, "HOME": str(tmp_path), "PYTHONPATH": str(Path.cwd())}
+    env = {
+        **os.environ,
+        "HOME": str(tmp_path),
+        "USERPROFILE": str(tmp_path),
+        "PYTHONPATH": str(Path.cwd()),
+    }
 
     with acquire_runtime_start_lock("default") as acquired:
         assert acquired is True
diff --git a/tests/test_platform_feature_matrix.py b/tests/test_platform_feature_matrix.py
new file mode 100644
index 000000000..9cf2511fb
--- /dev/null
+++ b/tests/test_platform_feature_matrix.py
@@ -0,0 +1,70 @@
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+MATRIX_PATH = Path("docs/platform-feature-matrix.json")
+VALID_STATUSES = {"covered", "partial", "gap", "blocked"}
+PLATFORMS = {"linux", "macos", "windows"}
+
+
+def test_platform_feature_matrix_is_complete() -> None:
+    matrix = json.loads(MATRIX_PATH.read_text(encoding="utf-8"))
+
+    assert matrix["schema_version"] == 1
+    assert set(matrix["platforms"]) == PLATFORMS
+    assert set(matrix["status_values"]) == VALID_STATUSES
+    assert matrix["features"], "matrix must list hardening features"
+
+    feature_ids: set[str] = set()
+    for feature in matrix["features"]:
+        feature_id = feature["id"]
+        assert feature_id not in feature_ids, f"duplicate feature id: {feature_id}"
+        feature_ids.add(feature_id)
+        assert feature["name"]
+        assert feature["risk"] in {"install", "runtime", "performance", "cache", "proxy"}
+        assert set(feature["platforms"]) == PLATFORMS
+
+        for platform, coverage in feature["platforms"].items():
+            status = coverage["status"]
+            assert status in VALID_STATUSES, f"{feature_id}/{platform} has invalid status"
+            assert coverage["tests"], f"{feature_id}/{platform} must cite tests or workflows"
+            for test_ref in coverage["tests"]:
+                path = Path(test_ref.split("#", 1)[0])
+                assert path.exists(), f"{feature_id}/{platform} cites missing path {test_ref}"
+            if status in {"partial", "gap", "blocked"}:
+                assert coverage.get("gap"), f"{feature_id}/{platform} must explain {status}"
+
+
+def test_platform_feature_matrix_covers_issue_1843_regression_areas() -> None:
+    matrix = json.loads(MATRIX_PATH.read_text(encoding="utf-8"))
+    feature_ids = {feature["id"] for feature in matrix["features"]}
+
+    assert {
+        "install_windows_service",
+        "single_instance_start",
+        "compression_fail_open",
+        "proxy_functional_smoke",
+        "ccr_persistence",
+        "toin_skip_recommendations",
+    } <= feature_ids
+
+
+def test_platform_feature_matrix_sanity_tests_are_enumerated() -> None:
+    matrix = json.loads(MATRIX_PATH.read_text(encoding="utf-8"))
+    sanity_ids = {item["id"] for item in matrix["sanity_tests"]}
+
+    assert {
+        "cli_help",
+        "install_paths",
+        "runtime_selection",
+        "health_startup",
+        "compression_backpressure",
+        "proxy_route_smoke",
+    } <= sanity_ids
+    for item in matrix["sanity_tests"]:
+        assert item["description"]
+        assert item["tests"]
+        for test_ref in item["tests"]:
+            path = Path(test_ref.split("#", 1)[0])
+            assert path.exists(), f"{item['id']} cites missing path {test_ref}"
diff --git a/tests/test_platform_stabilization_functional.py b/tests/test_platform_stabilization_functional.py
new file mode 100644
index 000000000..f89b1a296
--- /dev/null
+++ b/tests/test_platform_stabilization_functional.py
@@ -0,0 +1,166 @@
+from __future__ import annotations
+
+import json
+import time
+from typing import Any
+
+import pytest
+
+pytest.importorskip("fastapi")
+pytest.importorskip("headroom._core")
+
+from fastapi.testclient import TestClient
+
+from headroom.config import TransformResult
+from headroom.proxy.server import ProxyConfig, create_app
+
+
+def _proxy_config(**overrides: Any) -> ProxyConfig:
+    defaults: dict[str, Any] = {
+        "optimize": True,
+        "cache_enabled": False,
+        "rate_limit_enabled": False,
+        "cost_tracking_enabled": False,
+        "log_requests": False,
+        "ccr_inject_tool": False,
+        "ccr_handle_responses": False,
+        "ccr_context_tracking": False,
+        "image_optimize": False,
+        "disable_kompress": True,
+        "compression_max_workers": 1,
+    }
+    defaults.update(overrides)
+    return ProxyConfig(**defaults)
+
+
+def test_proxy_health_surfaces_compression_runtime_metrics(monkeypatch) -> None:
+    monkeypatch.setenv("HEADROOM_SKIP_UPSTREAM_CHECK", "1")
+    app = create_app(_proxy_config(optimize=False))
+
+    with TestClient(app, base_url="http://127.0.0.1", client=("127.0.0.1", 12345)) as client:
+        live = client.get("/livez")
+        health = client.get("/health")
+
+    assert live.status_code == 200
+    assert live.json()["alive"] is True
+    assert health.status_code == 200
+    runtime = health.json()["runtime"]
+    assert runtime["compression_executor"]["max_workers"] == 1
+    assert runtime["compression_executor"]["queued"] == 0
+    assert runtime["compression_executor"]["queue_timeouts_total"] == 0
+
+
+def test_v1_compress_success_reports_actual_metrics(monkeypatch) -> None:
+    monkeypatch.setenv("HEADROOM_SKIP_UPSTREAM_CHECK", "1")
+    app = create_app(_proxy_config())
+    proxy = app.state.proxy
+    request_messages = [{"role": "user", "content": "summarize this repeated payload"}]
+    compressed_messages = [{"role": "user", "content": "summary payload"}]
+
+    def fake_apply(**kwargs):
+        assert kwargs["messages"] == request_messages
+        assert kwargs["model"] == "gpt-4o"
+        return TransformResult(
+            messages=compressed_messages,
+            tokens_before=100,
+            tokens_after=40,
+            transforms_applied=["test:compress"],
+            markers_inserted=["marker-1"],
+        )
+
+    monkeypatch.setattr(proxy.openai_pipeline, "apply", fake_apply)
+
+    with TestClient(app, base_url="http://127.0.0.1", client=("127.0.0.1", 12345)) as client:
+        response = client.post(
+            "/v1/compress",
+            json={"model": "gpt-4o", "messages": request_messages},
+        )
+
+    body = response.json()
+    assert response.status_code == 200
+    assert body["messages"] == compressed_messages
+    assert body["tokens_before"] == 100
+    assert body["tokens_after"] == 40
+    assert body["tokens_saved"] == 60
+    assert body["compression_ratio"] == 0.4
+    assert body["transforms_applied"] == ["test:compress"]
+    assert body["transforms_summary"] == {"test:compress": 1}
+    assert body["ccr_hashes"] == ["marker-1"]
+
+
+def test_v1_compress_timeout_fails_open_quickly(monkeypatch) -> None:
+    monkeypatch.setenv("HEADROOM_SKIP_UPSTREAM_CHECK", "1")
+    app = create_app(_proxy_config())
+    proxy = app.state.proxy
+    request_messages = [{"role": "user", "content": "do not mutate me"}]
+
+    async def timeout_executor(fn, *, timeout):  # noqa: ANN001
+        raise TimeoutError("compression deadline exceeded")
+
+    monkeypatch.setattr(proxy, "_run_compression_in_executor", timeout_executor)
+
+    with TestClient(app, base_url="http://127.0.0.1", client=("127.0.0.1", 12345)) as client:
+        started = time.perf_counter()
+        response = client.post(
+            "/v1/compress",
+            json={"model": "gpt-4o", "messages": request_messages},
+        )
+        elapsed = time.perf_counter() - started
+
+    body = response.json()
+    assert response.status_code == 200
+    assert elapsed < 0.5
+    assert body["messages"] == request_messages
+    assert body["tokens_saved"] == 0
+    assert body["compression_ratio"] == 1.0
+    assert body["transforms_applied"] == []
+    assert body["compression_skipped"] is True
+    assert body["skip_reason"] == "compression_timeout"
+
+
+def test_v1_compress_real_json_tool_payload_reduces_tokens(monkeypatch) -> None:
+    monkeypatch.setenv("HEADROOM_SKIP_UPSTREAM_CHECK", "1")
+    app = create_app(
+        _proxy_config(
+            ccr_inject_marker=False,
+            min_tokens_to_crush=20,
+            max_items_after_crush=10,
+        )
+    )
+    items = [
+        {
+            "id": i,
+            "status": "ok",
+            "score": i % 5,
+            "message": "same repeated value " * 20,
+        }
+        for i in range(80)
+    ]
+    request = {
+        "model": "gpt-4o",
+        "messages": [
+            {"role": "user", "content": "summarize rows"},
+            {
+                "role": "assistant",
+                "content": None,
+                "tool_calls": [
+                    {
+                        "id": "call-1",
+                        "type": "function",
+                        "function": {"name": "list_rows", "arguments": "{}"},
+                    }
+                ],
+            },
+            {"role": "tool", "tool_call_id": "call-1", "content": json.dumps(items)},
+        ],
+    }
+
+    with TestClient(app, base_url="http://127.0.0.1", client=("127.0.0.1", 12345)) as client:
+        response = client.post("/v1/compress", json=request)
+
+    body = response.json()
+    assert response.status_code == 200, response.text
+    assert body["tokens_before"] > body["tokens_after"], body
+    assert body["tokens_saved"] > 0
+    assert body["compression_ratio"] < 1.0
+    assert body["transforms_applied"], body